This invention relates to a method of removing minute suspended solid particles from a large volume of liquid, and more particularly to a method of recovering coal fines from coal slurry.
The increasing importance of coal as an energy resource to replace the higher price and political vulnerability of oil supplies has spurred conversion from oil fired to coal fired power plants. However, the transportation and environmental problems inherent in the use of coal have introduced significant delays in achieving full utilization of this abundant resource.
Coal slurry is the key to the solution of the transportation and environmental problems. To form a slurry, the coal is accurately ground in a precision crusher, such as the CAGE-PACTOR from Gundlach Machine Co. in Belleville, Ill., and is mixed with sufficient water to produce a slurry. In slurry form, the coal can be cleaned to remove clay and other contaminants, if necessary, and it can be pumped directly over long distances in a pipeline. Coal slurry pipelines are an efficient way of transporting large volumes of coal over long distances. They are especially attractive as a dedicated supply line to a large power complex because the supply is continuous and reliable, as proven by the Black Mesa to Mojave System, now in operation over ten years.
A particularly troublesome aspect of coal slurry operations has been the necessity of dewatering the coal slurry to sufficient dryness so that it may be conveniently handled and stored, and economically burned in the boiler. Slurry is normally dewatered in a two step process, known as primary and secondary dewatering. Primary dewatering is typically accomplished on a vacuum filter, or on a centrifuge, via filtration or centrifugation. These devices can process large volumes of coal slurry and produce a coal cake containing about 20% water. This cake can be pulverized, dried, and fed directly to the boiler. The primary dewatering devices have only one serious shortcoming: they fail to remove the fine coal particles from their filtrate or effluent. Indeed, this primary filtrate or effluent contains a significant amount of coal, often in the range of 5-8% coal by weight. It is actually a dilute coal slurry itself, which I will call the "secondary slurry" stream to distinguish from the "primary slurry" stream received by the primary dewatering device.
The secondary slurry has proven to be a difficult substance to handle. It is so difficult, in fact, that in some installations it is simply wasted to large lagoons where both the coal and the water are lost. However, the coal and water in the "secondary slurry" are both valuable resources, and the cost of these lost resources and the cost of building lagoons makes this solution a very undesirable one. Alternately, merely recirculating the secondary slurry back into the primary slurry stream is not appropriate because it hydraulically overloads the primary dewatering device. The third and heretofore most effective technique is to process the secondary slurry through a gravity clarifier which allows the coal fines to settle to the bottom of a large basin where they are drawn off as an "underflow" and fed directly to the boiler. This technique does recover the coal and the water, but it is wasteful of the heating value of the coal since large amounts of energy must be expended to vaporize the water in the clarifier underflow which usually exceeds 80%.
To concentrate the solids content of the clarifier underflow or centrate and reduce its water content, several secondary mechanical dewatering or filtering devices such as the belt press or the filter press have been tried. However, it has been found that the solids content in the underflow often produced a gummy, sticky cake that sticks to the filter media and produces such severe handling problems that the use of these devices to dewater the centrate or underflow was considered inappropriate.
Thus, there existed prior to this invention, a serious need for an efficient and effective method for recovering substantially all of the coal fines from the secondary slurry and purifying the water therein to sufficient purity that it can be used in plant processes or returned to the environment.